1. Field of the Invention
This invention relates to a multilayer-wiring substrate for surface-mounting electronic and/or mechanical components, and particularly to such a substrate and method for making the same wherein at least two wiring conductors that sandwich an insulating layer are electrically connected by a via-hole conductor penetrating the insulating layer. More specifically, the present invention relates to a method for fabricating a multilayer-wiring substrate having wiring conductors formed alternatingly with insulating resin layers and having a via-conductor penetrating through the insulating resin layer(s) for establishing electrical continuity between the wiring conductors separated by the insulating layer(s).
2. Description of the Related Art
Some conventional steps for fabricating a multilayer-wiring substrate are described hereinafter referring to FIGS. 5(a)-5(b). A wiring conductor 14 is formed on a surface of an insulating resin layer 12 and then a photosensitive-resin layer 13 is coated on the insulating resin layer 12 and the wiring conductor 14. Next, the photosensitive-resin insulator layer 13 is exposed to light through a photomask and is then developed to thereby form a via-hole 15 so as to expose a surface of the wiring conductor, based on a photolithography method. Subsequently, another wiring conductor 18 is formed on a surface of the insulating resin layer 13 by, e.g., electroless copper plating or copper electroplating. At the same time, a via-conductor 17 is formed along an inner wall of the via-hole 15 to establish electrical connection between these wiring conductors 14,18, as seen in FIG. 5(b).
However, uniform development of a number of via-holes to be formed over an entire surface of a multilayer-wiring substrate by the photolithography method is difficult. In a certain portion of the multilayer-wiring substrate the via-holes may be accurately formed in the substrate, whereas, in another portion of the multilayer-wiring substrate, resin to be removed may remain on the bottoms of the via-holes due to inconsistent development speed of the photolithography. During the development of the via-hole, the resin which has once been dissolved in developer may accumulate on the bottom of the via-hole to prevent smooth feeding of new developer to the bottom of the via-hole. As a result, inward progress of development is hindered in a depth direction, resulting in insufficient development. As a result of such insufficient lithography, as shown in FIG. 5(c), the lower conductor 14 is not exposed through a bottom resin 15A of the via-hole 15. As a result, the lower wiring conductor 14 and the upper conductor wiring layer 18 are not connected by the via-conductor 17.
The bottom resin 15A has to be removed before forming the via-conductor, for example, by etching using a resin etchant such as a potassium permanganate solution. Even so, the bottom resin 15A is not removed completely and remains in a fragmental form in the via-hole 15, causing poor electrical continuity by way of the via-conductor between the lower conductor 14 and the upper conductor 18.
The present invention has been accomplished in view of the foregoing.
An object of the present invention is to provide a multilayer-wiring substrate with a via-conductor having a complete electrical connection and/or a lowest electrical impedance across lower and upper conductors separated by an insulating layer.
Another object of the invention is to provide a method for fabricating a multilayer-wiring substrate with a via-conductor having a uniform electrical continuity from a upper wiring conductor formed on a surface of an insulating layer to a lower wiring conductor formed on another surface of the insulating layer.
Still another object of the invention is to provide a method for completely removing undesired material adhered onto a wiring conductor that forms a bottom of a via-hole penetrating through an insulating resin layer so as to sufficiently expose the wiring conductor on which a solder bump or other via-conductor may be assuredly formed.
A further object of the invention is to provide a via-conductor structure penetrating straightly through at least two insulating layers of a multilayer wiring substrate.
According to a first aspect of the invention, a multilayer-wiring substrate is provided comprising:
a wiring conductor with a recess formed by etching a surface of the wiring conductor,
an insulating layer formed on a surface of the wiring conductor except over the recess;
another insulating layer formed on the other surface of the wiring conductor;
a via-hole penetrating through the insulating layer to the recess; and
a via-conductor or a solder bump formed inside the via-hole and adhered to a recessed surface of the wiring conductor.
An advantage of this multilayer wiring substrate is that an area of the wiring conductor to which a via-conductor or a solder bump adheres is increased by the recess. In other words, since a recessed surface is formed at the wiring conductor, a strong mechanical and electrical connection is attained between the wiring conductor and the via-conductor or the solder bump. The via-conductor or the solder bump is formed in the via-hole of the insulating layer and partly in the recess of the wiring conductor.
When a depth of the recess is 5-30% of a thickness of the wiring conductor, a good result is attained in such a connection. Further, when the recess is made by chemically etching the wiring conductor, a better connection strength of the via-conductor or solder bump to the wiring conductor is attained.
When the via-conductor is made by plating copper onto an inner peripheral wall of the via-hole that penetrates the insulating layer and by extendingly plating copper on an etched and recessed portion of the wiring conductor at the via-hole bottom, the best electrical connection is attained between the electrical conductors sandwiching the insulating layer. A via-conductor is formed by plating metal such as copper onto the recessed wiring conductor whereat the via-hole forms a bottom, and the via-conductor may be formed by further plating metal extendingly on an inner peripheral wall of the via-hole. The via-hole connects the wiring conductor with other electrical conductors formed on the other surface of the insulating layer.
The above mentioned multilayer-structure may be supported on an insulating core plate having a thickness of 0.5-1 mm, since the conductor, e.g., a copper layer, is about 10-25 xcexcm in thickness and the insulating layer coated on the copper is about 30-60 xcexcm in thickness. A plurality of the structures may be formed on the core plate, with alternatingly forming the wiring conductor and the insulating layer. Similar structure(s) may also be advantageously formed on the other side of the core plate in order to prevent warpage of the multilayer wiring substrate.
According to a second aspect of the invention, a method is provided for fabricating a multilayer-wiring substrate having a via-conductor in a via-hole, comprising:
forming an insulating layer on a wiring conductor; forming a via-hole in the insulating layer by removing the insulating layer to an extent that the insulating layer in the via-hole becomes fragmented and adheres to the wiring-conductor located at the bottom of the via-hole; then etching the wiring conductor located at the bottom of the via-hole so that the fragments are removed and a recess is formed in the conductor; and then forming a via-conductor in the via-hole by plating metal on an inner peripheral wall of the via-hole and extendingly plating the metal on a surface of the recessed portion of the wiring conductor.
Forming the via-hole in the insulator may be done by developing a photosensitive resin layer using a lithography technique or by laser-drilling the insulating layer.
An important step in the above described method according to the invention is etching the wiring conductor located at the bottom of the via hole after the via hole is made by photolithography development or by laser-drilling, so as to form the recess in the insulating layer. The photolithography can not completely remove the photosensitive resin layer stuck to the conductor layer by adhesion that will remain as undesired material at the via-hole bottom. Although the laser can drill the insulating layer easily, it leaves the undesired material carbonized or melted from the insulating layer on the bottom surface and the inner via-hole wall. Therefore, forming a recessed surface at the conductor layer surface after via-hole formation means perfect elimination of the undesired material from the surface of the via-conductor whereat the via-conductor is next to be formed.
When a plurality of the substrate structures are multilayered, a via-hole formed in the upper insulating layer is connected preferably in a concentric or coaxial position with the via-hole formed in the neighboring lower insulating layer. This enables one to electrically connect a via-conductor formed in a upper insulating layer with a via-conductor formed in a lower insulating layer in a shortest length with a smallest electrical impedance. The via-conductors can be aligned straightly through these via-holes in a thickness direction of the substrate, namely, in a perpendicular direction to the plane of the substrates, rendering the lowest electrical impedance across the insulating layers. When the via-conductor in the lower insulating layer is made by plating copper on the inner peripheral wall of the via-holes and on the recessed portion of the lower conductor, it is advantageous to fill the via-hole with the copper plating because a next insulating layer may be coated on the via-conductor which becomes a via hole bottom of the next via-hole that will be made through the next insulating layer. The via-hole bottom should be etched so as to form a recessed end surface of the via-conductor, to which recessed end surface a next via hole or a solder bump is connected.
A via-hole for forming a solder bump therein may be made in an outermost insulating layer of the multilayer-wiring substrate, in a manner penetrating through the outermost resin layer to an inner wiring copper layer formed at the inner side of the outermost resin layer. The surface of the inner wiring copper layer that forms a bottom of the via-hole may be plated with another metal such as gold and/or nickel so as to facilitate formation of the solder bump and to fix the solder bump firmly in the via-hole. An electrical continuity between the bottom metal of the via-hole and a terminal of a different electronic or other mechanical component is therefore accomplished by the solder ball formed inside the via-hole, regardless of whether the inner peripheral wall of the via-hole is plated or not.
An important feature of the above method of the present invention is that the fragments adhering to the conductor that forms a bottom of the via-hole is removed together with a portion of the conductor, by chemically etching the conductor on which the fragments are carried on. In other words, when the bottom conductor is chemically etched using an etchant, the etchant etches not only in the thickness direction but also in a planar direction of the bottom conductor and undercuts the conductor underneath the remainder, resulting in forming a recess in the bottom conductor as well as eliminating the fragments. Accordingly, the surface of the recessed area of the bottom conductor is cleaned up and the surface of the recessed conductor can be plated with a metal conductor such as copper, assuring that reliable electrical continuity and mechanical adhesion strength are established between upper and lower wiring conductors sandwiching the insulating layer.
According to a third aspect of the invention, a method is provided for fabricating a multilayer-wiring substrate, comprising:
forming a photosensitive resin layer on a wiring conductor; exposing the photosensitive resin layer to light through a photomask; making a via-hole in the photosensitive resin layer by developing the portions of the photosensitive resin exposed to light; chemically etching the photosensitive resin layer to an extent that the photosensitive resin layer becomes fragmented in the via-hole; undercutting the fragmentally remaining photosensitive resin at the bottom of the via-hole by chemically etching the wiring conductor that is exposed through the fragmentally remaining resin; forming a recess in the conductor by further chemically etching the wiring conductor, and forming a via-conductor in the via-hole by plating metal on an inner peripheral wall of the via-hole and on a surface of the wiring conductor that is chemically etched and recessed.
A feature of the above method according to the third aspect of the present invention is as follows. By chemically etching the surface of the conductor exposing through the fragmental remainder such as resin carried on the bottom of the via-hole, such remainder remaining partly or rather fragmentarily at the bottom of the via-hole can be undercut and removed together with a portion of the conductor. In other words, when the bottom conductor is chemically etched using an etchant, the etchant etches not only in the thickness direction but in a planar direction of the bottom conductor underneath the remainder, resulting in forming a recess at the bottom conductor. Accordingly, the surface of the recessed area of the bottom conductor is cleaned up and the surface of the recessed conductor can be plated with a metal conductor such as copper. This assures that reliable electrical continuity and mechanical adhesion strength are established by the via-hole penetrating the insulating layer. An upper conductor may be formed on the outermost insulating layer, which conductor may serve as a bonding pad for electrical connection from the inner wiring conductor to, e.g., a signal transmission terminal of a semiconductor chip.
The above method according to the third aspect may further comprise:
forming a second photosensitive resin layer on a second wiring conductor; exposing the second photosensitive resin layer to light through another photomask; making a second via-hole in the second photosensitive resin layer by developing the portions of the second photosensitive resin exposed to light; chemically etching the second photosensitive resin layer to an extent that the second photosensitive resin layer becomes fragmented in the second via-hole; undercutting the fragmentally remaining second photosensitive resin at the bottom of the second via-hole by chemically etching the second wiring conductor that is exposed through the fragmentally remaining resin; forming a recess in the second conductor by further chemically etching the second wiring conductor; and plating a metal on a surface of the second wiring conductor that is chemically etched and recessed.
A solder bump may be formed by soldering to a recessed area of the second wiring conductor and held in the second via-hole, for electrically connecting the second wiring conductor with an electronic component terminal outside.
An alternative way of making a via-hole in an insulating layer is to use a laser.
When the laser is used for making a via-hole in the insulating layer, the laser melts or burns some of the insulating layer material to be carbonized. In the case that a resin is used for the insulating layer, the carbonized material formed from the resin due to a burning action of the laser adheres to a surface of the conductor at the via-hole bottom. In the case that an inorganic insulating material such as glass, ceramic and a mixture thereof is used for the insulating layer, the laser melts the inorganic insulating material which is splashed during formation of the via-hole onto a surface of the conductor. The present invention teaches that removal of any undesired material adhered on the surface of the conductor at the via hole bottom is necessary before forming a via-conductor in the via-hole for electrical connecting the upper and lower conductors sandwiching the insulating layer(s), and provides a method for removing such undesired material by chemically etching the conductor existing underneath the lower conductor. A recess should be formed in the wiring conductor formed on the insulating layer and extendingly formed on the end of the via-conductor so as to assure such electrical connection by the via-conductor between upper and lower wiring conductors. When a depth of the recess becomes 5-30% of the thickness of the wiring conductor that is normally in the range of 10-25 xcexcm in thickness, the best electrical connection is attained, according to the invention.
In a case of making a multilayer-wiring substrate having inorganic insulating layers made of ceramic, glass-ceramic or glass alternated with metal-wiring layers made of at least one of copper, nickel, gold, silver, palladium, tungsten and molybdenum, a method of making such an inorganic multilayer-substrate may include a step of co-firing green (unfired) insulating layers and metal-wiring layers to form a fired multilayer-wiring substrate, before forming a solder bump on a multilayer-wiring substrate.